The approaches described in this section could be pursued but are not necessarily approaches that have previously been conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
PFS is a property of key-agreement protocols ensuring that compromising of a session key derived from long-term keys is impossible even if one of the long-term keys is compromised in the future. By using a key-agreement protocol, two or more parties can agree on the session key in such a way that all parties influence the generated session key. The PFS can be used in the Secure Sockets Layer (SSL) cryptographic protocol. The SSL protocol may use asymmetric cryptography to authenticate the counterparty with whom the protocol is communicating. The asymmetric cryptography is a cryptographic algorithm that requires two separate keys, referred to as a private key and a public key, to encrypt and decrypt data flowing between parties. The private key and the public key can be mathematically linked so that encryption of an encryption key, also referred to as a session key, by one party using the public key allows decryption of the session key by another party using the private key. Therefore, before beginning to exchange information protected by the SSL protocol, a client and a server must securely exchange or agree upon the session key to use when encrypting data flowing between the client and the server.
SSL sessions between the client and the server commence with a PFS handshake procedure that includes a process of negotiation that dynamically sets parameters of a communications channel established between the client and the server. Some steps of the handshake procedure may be very expensive by requiring the server to perform time and resource consuming computations to generate a public key for transmission to the client. An attacker may take advantage of such workload on the server and send multiple session requests to the server without any intent to establish a valid session. The attacker can simply terminate the connection after receiving a public key generated by the server and immediately reconnect with a new request. Alternatively, the attacker may respond to the server by sending random numbers instead of generating and encrypting a valid session key based on the received public key. As such actions of the attacker can easily overwhelm the capacity of the server or interrupt proper functioning of the server, they can be used in a denial of service (DoS) attack or, in case of distributed attackers, a Distributed DoS (DDoS) attack on the server.